1. Field of the Invention
The invention relates generally to communication systems, and more particularly to an apparatus and method for achieving synchronization in digital receivers used in communication systems.
2. Related Art
In synchronous digital transmission, information is conveyed by uniformly spaced pulses and the function of any receiver is to isolate these pulses as accurately as possible. However, due to the noisy nature of the transmission channel, the received signal has undergone changes during transmission and an estimation of certain reference parameters is necessary prior to data detection. Estimation theory proposes various techniques for estimating these parameters depending on what is known of their characteristics. One such example technique is called maximum likelihood (ML). A maximum likelihood estimation assumes the parameters are deterministic or, at most, slowly varying over the time interval of interest. The term deterministic implies the parameters are unknown but of a constant value and are, therefore, not changing over time. These unknown parameters can cover factors such as the optimum sampling location, the start of a packet (or a frame marker for continuous data streams) or the phase offset introduced in the channel or induced by instabilities between the transmitter and receiver oscillators. It is widely recognized that maximum likelihood estimation techniques offer a systematic and conceptually simple guide to the solution of synchronization problems. Maximum likelihood offers two significant advantages: it leads to appropriate circuit configuration and provides near optimum or optimum performance depending on the known channel conditions (J. G. Proakis, xe2x80x9cDigital Communications,xe2x80x9d Third Edition, McGraw-Hill Publishers, pp. 333-336, 1995).
Generally, if the transmitter does not generate a pilot synchronization signal, the receiver must derive symbol timing from the received signal. The term symbol is used in this context to refer to transmitted signals that are phase modulated with discrete phase relationships, where each assigned phase relationship is a symbol that is subject to detection at the receiver. Both the transmitter and receiver employ separate clocks which drift relative to each other, and any symbol synchronization technique must be able to track such drift. Therefore choosing the proper sampling instants for reliable data detection is critical, and failure to sample at the correct instants leads to Inter-Symbol Interference (ISI), which can be especially severe in sharply bandlimited signals (M. H. Meyers and L. E. Franks, xe2x80x9cJoint Carrier Phase and Symbol Timing Recovery for PAM Systems,xe2x80x9d IEEE Transactions on Communications, COM-28(8):1121-1129, 1977). The term ISI refers to two or more symbols that are superimposed upon each other; phase detection of each symbol, thus, becomes extremely difficult. Incorrect sampling implies the receiver is inadvertently sampling the signal where the influence of the previous data symbol is still present (J. G. Proakis, xe2x80x9cDigital Communications,xe2x80x9d Third Edition, McGraw-Hill Publishers, pp. 536-537, 1995).
In a receiver, the signal following demodulation is first passed through a matched filter and sampled. The optimum sampling times correspond to the maximum eye opening and are located approximately at the peaks of the signal pulses. The term xe2x80x9ceye openingxe2x80x9d refers to the amplitude variations of the signal at the output of the pulse-shaping filter. An eye is formed by superimposing the output of the pulse shaping filter for each symbol upon the other until the central portion takes on the shape of an eye as illustrated in FIGS. 10a and 10b. Note that at high signal to noise conditions, the xe2x80x9ceyexe2x80x9d is open, whereas at low signal to noise conditions, the xe2x80x9ceyexe2x80x9d is closed.
Among synchronization systems, a distinction is made between feedforward and feedback systems. A feedback system uses the signal available at the system output to update future parameter estimates. Feedforward systems process the received signal to generate the desired estimate without explicit use of the system output. A particular form of feedback system is data directed. Data directed techniques make explicit use of prior data decisions to estimate the current estimate of the unknown parameter, which is then used to update the data decisions (J. G. Proakis, xe2x80x9cDigital Communications,xe2x80x9d Third Edition, McGraw-Hill Publishers, pp. 333-336, 1995). Whether the design approach is feedforward or feedback, both techniques are related to the maximum likelihood parameter estimation. Both feedforward and feedback techniques are being used in the current technology. However, it should be noted that there are advantages and disadvantages associated with both approaches. The disadvantages of feedback techniques are well documented in the literature (U. Mengali and N. D""Andrea, xe2x80x9cSynchronization Techniques for Digital Receivers,xe2x80x9d Plenum Press Publishers, p. 398, 1998).
There are two alternatives to receiver design namely, coherent demodulation and non-coherent demodulation. Coherent demodulation is used when optimum error performance is essential. This implies that the baseband data signal is derived making use of a local reference with the same frequency and phase as the incoming carrier. This requires accurate frequency and phase measurements insofar as phase errors introduce crosstalk between the in-phase and quadrature channels of the receiver and degrade the detection process. The extraction of the phase occurs in a process termed phase estimation. Furthermore, frequency estimation is necessary when the local receiver oscillator and the received signal frequency differ in frequency and phase by a sufficient amount such that phase recovery is not sufficient to ensure reliable data detection. Phase recovery algorithms have a limited pre-tracking ability, and when the phase offset exceeds the tracking ability of the phase recovery circuitry, frequency estimation becomes necessary. Depending on the phase/frequency offset present, frequency estimation algorithm have a much wider tracking range than their phase tracking counterparts. In fact frequency estimation is generally done first and followed by phase estimation.
An alternative receiver design approach is to use non-coherent demodulation techniques, such as differential demodulation where the phase difference between one data symbol and the next is assumed constant. In applications where simplicity and robustness of implementation are more important than achieving optimum performance, differentially coherent and non-coherent demodulation are attractive alternatives to coherent demodulation.
The effect of a poorly designed carrier loop increases the dispersion of the received symbols about their nominal values, bringing the received points considerably closer to the decision boundaries and decreasing the error margin. Of course, large phase perturbations can cause errors without any noise. Similarly, timing phase errors will cause the receiver to sample away from the maximum eye opening and reduce the margin for error. In traditional analog implemented receivers, synchronization is typically performed using an error tracking synchronizer whereby a feedback loop constantly adjusts the phase of a local clock to minimize the error between the estimated and the optimum sampling instant. Flexibility in the design of the synchronization unit in a receiver has increased in recent times with the advent of increasingly powerful silicon chips. This has led to the adoption of open loop (otherwise known as feedforward) estimation techniques for synchronization purposes.
Digital synchronization methods recover timing, phase and frequency estimates by operating only on signal samples taken at a suitable rate. FIGS. 11a and 11b illustrate the concept of sampling a signal. FIG. 11a illustrates that when oversampling occurs at a rate of four samples per symbol, the information available with regard to the received signal is much greater than that in FIG. 11b. This is in contrast to analog methods that operate on continuous time waveforms.
Digital circuits have an enormous appeal in communications technology and influence the design of all modern receivers. The advantages of a digital implementation are that it does not require re-alignment, it has less stringent tolerances, low power consumption and can easily be integrated into a low cost component.
With the advent of digital receiver design, there has been an increase in the development of techniques with no counterpart in analog receiver design. The most significant example of this is the application of interpolation and decimation to receiver design. This trend is in response to the increased use of all digital receivers where sampling of the received signal is asynchronous to the incoming data symbols. In such receivers, timing adjustment is done after sampling using data interpolation and decimation. To date, interpolation has been used to estimate the signal value at the optimum sampling instant using a timing offset which has been previously calculated using some established timing estimation algorithm {(F. M. Gardner, xe2x80x9cA BPSK/QPSK Timing Error Detector for Sampled Receivers,xe2x80x9d IEEE Transactions on Communications, 34:423-429, May 1986) and (H. Meyr, M. Moeneclaey and S. A. Fechtel, xe2x80x9cDigital Communication Receivers: Synchronization, Channel Estimation and Signal Processingxe2x80x9d, John Wiley publishers, pp. 289-295, 1998)}. Furthermore, to remove the redundant sampling instants produced by the asynchronous sampling of the received signal, a decimator follows the interpolator. The interpolator is essentially a rate conversion mechanism whereby the signals at the input and output are operating at two distinct, yet unrelated, sampling rates. In a feedforward arrangement, sampling is typically not directly synchronized to the data symbols, and subsequent processing must choose the optimum sampling instant without the luxury of altering the phase of the sampling clock (F. M. Gardner, xe2x80x9cInterpolation in Digital Modems-Part 1: Fundamentals,xe2x80x9d IEEE Transactions on Communications, 41:501-507, March 1993). This problem has been overcome by the application of interpolation. Interpolation estimates the optimum sampling instant from the signal values obtained by oversampling the received signal. The use of interpolation is not a new development and has been applied for many years in signal processing (R. E. Crochiere and L. R. Rabiner, xe2x80x9cInterpolation-Decimation Circuit for Increasing or Decreasing Digital Sampling Frequency,xe2x80x9d U.S. Pat. No. 4,020,332, issued Apr. 26, 1977). However, its applicability to receiver synchronization is a recent development {(R. De Gaudenzi, M. Luise and R. Viola, xe2x80x9cA Digital Chip Timing Recovery Loop for Band-Limited Direct Sequence Spread Spectrum Signals,xe2x80x9d IEEE Transactions on Communications, 41(11):1760-1769, 1993) and (H. Meyr, M. Moeneclaey and S. A. Fechtel, xe2x80x9cDigital Communication Receivers: Synchronization, Channel Estimation and Signal Processingxe2x80x9d, John Wiley publishers, pp. 289-295, 1998)}.
For synchronization purposes, the interest is not in regenerating the original analog waveform as is common in some traditional applications of interpolation to signal processing. Instead, only the optimum sampling instant corresponding to the location of the maximum xe2x80x9ceye openingxe2x80x9d is required. The maximum eye opening is where the output rate can not be considered as a multiple of the original sampling rate (F. M. Gardner, xe2x80x9cInterpolation in Digital Modems-Part 1: Fundamentals,xe2x80x9d IEEE Transactions on Communications, 41:501-507, March 1993). The interpolated sample values can be computed entirely from knowledge of the input data sequence, the interpolating filter impulse response and the time instants of the input and output samples. To date many approximations have been suggested for the ideal interpolating filter response, the most popular shapes being linear, parabolic, piecewise parabolic, cubic and spline (L. Erup, F. M. Gardner and R. A. Harris, xe2x80x9cInterpolation in Digital Modems-Part II: Implementation and Performance,xe2x80x9d IEEE Transactions on Communications, 41:998-1008, June 1993).
Frame synchronization is defined as determining the start of a predefined frame marker (or unique word) in a stream of continuously transmitted frames. This predefined data pattern is commonly referred to as a frame marker, a unique word, or a frame preamble, e.g., a two symbol Barker code, and is inserted into the data stream at the transmitter to indicate either the start of a discontinuous data stream, known as a packet, or as a marker inserted in a continuous data stream to keep track of the data. Reliable frame synchronization requires careful selection of the unique word to minimize correlation sidelobes and thereby the probability of false synchronization. The data immediately surrounding the unique word affects this selection. The unique word must not only have low aperiodic auto-correlation sidelobes to minimize the false synchronization probability with noise or random data, but also have low aperiodic auto-correlation sidelobes when preceded by a preamble. More specifically, if such a synchronization sequence is correlated with itself, the correlator generate a pulse output only when the sequences being correlated are aligned. At other times, the correlator""s output is zero or nearly zero.
The methods described in the literature present many problems, such as complexity in having four or more samples per symbol for feedforward timing estimation. Therefore, at high data rates, feedforward parameter estimation requires a high data clock to sufficiently oversample the received signal for the established techniques in the literature. Another problem with feedback techniques used in the prior art is the acquisition time as well as the high probability of hangup and cycle slips associated with their phase locked loop (PLL) based structures (J. G. Proakis, xe2x80x9cDigital Communications,xe2x80x9d Third Edition, McGraw-Hill Publishers, Chapter 6, pp. 333-373, 1995), especially in the presence of fading associated with the mobile communications channel. The term xe2x80x9changupxe2x80x9d refers to the situation where the initial phase error is close to 180 degrees, which can result in an extremely long transient time. In fact, there are cases where the loop may never recover from xe2x80x9changup.xe2x80x9d The issue of xe2x80x9changupxe2x80x9d is very serious as it may occur under perfect noise conditions. Special circuits can be used to detect hangup and pull the PLL into its lock range. However, these are notoriously difficult to design for reliable operation. The term xe2x80x9ccycle slipxe2x80x9d refers to when the PLL oscillator drops or adds one oscillation cycle with reference to the input signal, which results in incorrect parameter estimation for a significant period depending on the estimator""s loop bandwidth due to the residual effect of the sharp error signal and the instability introduced into the loop by such an event. These issues can have a detrimental effect on the receiver""s symbol error rate and are solely due to the feedback nature of the traditional estimators.
These problems can be circumvented through the use of feedforward estimation. However, feedforward estimation techniques, in general, require a higher oversampling ratio than is prevalent in digital versions of analog feedback estimators (F. M. Gardner, xe2x80x9cA BPSK/QPSK Timing Error Detector for Sampled Receivers,xe2x80x9d IEEE Transactions on Communications, 34:423-429, May 1986). Furthermore, in the current technology, feedforward algorithms for timing estimation require at least four samples per symbol for reliable operation. Additionally, the current technology does not exploit the information from the frame synchronization output to estimate both phase and timing offsets in digital receivers.
Consequently, what is desired is to provide both phase and timing estimation based on the information obtained from the frame synchronization in a digital receiver. A method for obtaining timing recovery and initial phase offset estimation by using an interpolator over the frame correlation output is needed. It is also desired to use the timing estimate to assist the data interpolation and decimation and to remove the residual phase offset remaining on the received signal. There is a need for a joint frame, timing offset and phase estimation method for any digital receiver within a communication system wherein a small number of samples per symbol, e.g., two, are employed.
One aspect of the invention includes a system and method of joint frame, timing and phase estimation for use in a digital receiver within a communication system. A synchronization block is provided, within a digital receiver, where the inputted data stream is processed using L samples per symbol and operates in a feedforward manner. This method exploits the information available from frame synchronization to estimate the timing offset and static phase offset introduced in the channel. The method uses a dynamic digital interpolator over the frame synchronization correlation output to estimate these unknown parameters. The phase estimate is then used to initially de-rotate the received signal to remove the effect of the static phase offset. The timing estimate is subsequently used by a data interpolator to estimate the value of the received signal corresponding to this sampling instant for each of the transmitted symbols. The redundant samples are removed by decimation. The decimated output signal is provided to a frequency and phase tracking circuit to remove any remaining phase or frequency offsets. The processed signal is then forwarded to other functional blocks such as a decoder, if source coding is used at the transmitter. This method is provided for a variety of digital receivers employing Code Division Multiple Access (CDMA), in which a transmitted signal is spread over a band of frequencies much wider than the minimum bandwidth required to transmit the signal, Time Division Multiple Access (TDMA), where the users share the radio spectrum in the time domain, Frequency Division Multiple Access (FDMA), where a user is allocated at least one unique frequency for communication without interference with users in the same frequency spectrum, or any combination of the above or other technologies.
In another aspect of the invention, a digital receiver system comprises a filtering block, a synchronization subsystem and other functional blocks. The filtering block comprises a pulse-shaping filter. The synchronization subsystem comprises a frame synchronization and detection block, a timing estimation and initial phase offset estimation block, a real-time phase and frequency tracking block, and an interpolator and decimator. The filtering block receives signals from an Intermediate Frequency (IF) block which have been demodulated to baseband. These signals may be sampled by an Analog to Digital Converter (ADC) with a fixed clock (sampling clock=46.7 MHz). It is necessary to note that the signals received by the filtering block within the digital receiver may not be sampled, and that the sampling may take place only after the filtering block. The outputted signals from the filtering block are then fed to the synchronization subsystem for further processing.
In another aspect of the invention, there is a synchronization circuit, comprising a frame synchronizer receiving a data stream that includes data frames; and a dynamic interpolation module receiving the data stream and generating an estimated timing offset and a timing corrected data stream, wherein the dynamic interpolation module includes a timing estimator that utilizes the output of the frame synchronizer to estimate a timing offset, and wherein the estimated timing offset may change for each frame of the data stream.
In another aspect of the invention, there is a method of synchronizing a digital communications receiver, comprising receiving a signal stream comprising a plurality of data frames, obtaining a frame synchronization signal from the signal stream, and dynamically interpolating a phase offset and a timing offset of the signal stream from the frame synchronization signal.
In another aspect of the invention, there is a method of receiving a signal stream in a digital communications receiver, comprising providing an analog signal stream, converting the analog signal stream to a digital signal stream, correlating a unique word with the digital signal stream, detecting a threshold to determine whether frame synchronization has occurred generating coefficients for timing estimation and phase estimation, estimating a timing offset and a phase offset for the digital signal stream, generating a timing corrected and initial phase corrected signal stream from the estimated timing offset, estimated phase offset and the digital signal stream, and removing residual phase offsets from the timing corrected and initial phase corrected signal stream.
In another aspect of the invention, there is a system for detecting data, comprising a frame synchronizer receiving a data stream, a timing and phase estimator that utilizes the output of the frame synchronizer to estimate a timing offset and a phase offset, a phase rotation module receiving the estimated phase offset and the data stream to perform an initial phase correction on the data stream using the estimated phase offset, an interpolation module receiving the estimated timing offset and the phase corrected data stream to generate a timing corrected data stream, and a phase tracking and data detection module receiving the timing corrected data stream to remove a residual phase offset and detect the data.
In another aspect of the invention, there is a synchronization circuit in a digital wireless receiver, comprising a frame synchronizer receiving a data stream, a timing and phase estimator that utilizes the output of the frame synchronizer to estimate a timing offset and a phase offset, a derotator receiving the estimated phase offset and the data stream to perform an initial phase correction on the data stream using the estimated phase offset, an interpolation module receiving the estimated timing offset and the phase corrected data stream to generate a interpolated data stream, and a phase tracking and data detection module receiving the interpolated data stream to remove a residual phase offset and synchronize the data in the data stream.
In another aspect of the invention, there is a digital communications receiver capable of receiving a signal stream having a plurality of data frames, wherein the signal stream is used to synchronize portions of the receiver, comprising a sampling circuit capable of sampling symbol levels in a synchronizing signal sequence of a data frame, a pulse shaping filter capable of receiving and filtering the sampled signal sequence, a frame synchronization circuit capable of receiving the filtered signal sequence and correlating a unique word with the filtered signal sequence, a threshold detection mechanism in communication with the frame synchronization circuit and capable of determining whether synchronization has occurred, an interpolation coefficient generator in communication with the frame synchronization circuit and capable of generating coefficients for timing estimation and phase estimation, a timing and phase estimator that utilizes the output of the frame synchronization circuit and of the interpolation coefficient module to estimate a timing offset and a phase offset a dynamic interpolator receiving the estimated timing offset, the estimated phase offset and the signal stream to generate a timing corrected signal stream, and a decision directed phase tracker capable of receiving the timing corrected signal stream removing residual phase offsets.